Accurate assessments of regional- and global-scale changes in the terrestrial
biosphere are essential if human impacts on biosphere-atmosphere function
are to be understood. There are a myriad of ecosystem attributes to
be monitored, but quantifying human impacts necessarily includes an
evaluation of vegetation cover and net primary productivity (NPP), as
these determine amounts of fuel, fiber, and food for human consumption
(Running et al. 1999). A global terrestrial observing system is needed
that integrates field-based measurements, flux towers, remote sensing,
and ecosystem modeling (Baldocchi et al. 1996, Running et al. 1999,
Canadell et al. 1999).
Ecosystem process models that simulate carbon, water, and energy exchange
between terrestrial ecosystems and the atmosphere require leaf area
index (LAI) and vegetation cover as primary drivers (Landsberg and Gower
1997, Waring and Running 1998), and these must be derived by remote
sensing. MODIS (Moderate Resolution Imaging Spectrometer) is the primary
high temporal frequency mapping sensor onboard NASA's Earth Observing
System (EOS) satellite Terra, launched in December 1999. MODIS is poised
to become the most important global mapping sensor ever, as it views
the entire Earth's surface every 1-2 days acquiring data in 36 spectral
bands at spatial resolutions of 250 to 1000m.
Validation of the global data products derived from MODIS and related
sensors is essential to both assess product accuracy and to provide
feedback to algorithm developers so the algorithms can be improved.
Faced with the challenge of validating global remotely sensed products,
NASA formed the EOS Validation Program to assist MODIS (and other) Science
and Instrument Teams with product validation. For the Land Science Team
(MODLand), research at intensive study sites forms the backbone of the
validation plan. These have evolved into what constitutes the
MODLand core validation sites network. The sites associated with
our current project, BigFoot, are important sites within that network.
Each BigFoot site is centered on an eddy flux tower that measures continuous
water, energy, and carbon fluxes that can potentially be used to validate
MODIS products. However, with their relatively small footprint on the
order of 1
km2, nearly equivalent to a single
MODIS resolution cell (that in most cases will not perfectly align with
the footprint), it is important that the spatial context of flux towers
BigFoot is designed to provide that context using a combination of in
situ ecological data, Landsat ETM+ data, and ecosystem models (Cohen
and Justice 1999). Moreover, BigFoot maps land cover, LAI, fraction
absorbed photosynthetic active radiation (fAPAR),
and NPP over a 5 x 5 km area around an eddy flux tower at ETM+ resolution.
This means we fully characterizes 25 MODIS cells around a given tower
site, and are able to test a number of scaling factors that should reveal
possible causes of MODIS mapping errors (thereby providing feedback
to algorithm developers). Finally, BigFoot takes important steps to
enhance the goals of GTOS (the Global
Terrestrial Observing System.